Use mutable date for Date base class.

Historically DateTimes from date columns were mutable.
Make that the default now as well.

src/Database/Type/DateType.php

@@ -15,7 +15,7 @@
 namespace Cake\Database\Type;
 
 use Cake\Database\Driver;
-use DateTimeImmutable;
+use DateTime;
 
 class DateType extends DateTimeType
 {

src/I18n/Date.php

@@ -14,7 +14,7 @@
  */
 namespace Cake\I18n;
 
-use Cake\Chronos\Date as BaseDate;
+use Cake\Chronos\MutableDate;
 use IntlDateFormatter;
 use JsonSerializable;
 
@@ -23,7 +23,7 @@
  *
  * Adds handy methods and locale-aware formatting helpers
  */
-class Date extends BaseDate implements JsonSerializable
+class Date extends MutableDate implements JsonSerializable
 {
     use DateFormatTrait;